1. Field of the Invention
The present invention relates to beam scanning antennas, and more specifically relates to waveguide scanners. Even more particularly, the present invention relates to variable width waveguide scanners.
2. Description of the Prior Art
Variable width waveguide scanning antennas are well known in the art. They were first developed during World War II and were commonly referred to Eagle scanners. A beam scan of .+-.30 .degree. was achieved with the original Eagle scanners by mechanical variation of the a-dimension of an end-fed waveguide from which microwave energy was coupled using probes to a series of radiating dipoles. The Eagle scanner was based on the principle that variation of the waveguide a-dimension caused a variation of velocity of propagation within the waveguide. This change in waveguide velocity resulted in a corresponding change in the element-to-element phase shift which "scanned" the beam.
The Eagle scanner described above was composed of two principal elements, as shown in FIG. 1. The first element is a movable "waveguide squeeze" section which produced the variation in the microwave propagation within the waveguide. This section employed two nested L-shaped sections 2 which were moved relative to each other (in the direction of the arrows) to reduce the effective a-dimension of the waveguide structure formed between them. Waveguide chokes in the form of quarter wave slots 4 were formed in the smaller leg portion 6 of each L-section to reduce leakage of microwave energy through the gaps between the two L-sections.
The second principal element of the basic Eagle scanner was the structure which was employed to couple energy from the waveguide squeeze section out to form the desired scanning beam. A row of waveguide probes 8 were employed for this purpose. The probes 8 were mounted in the larger leg portion 10 of one of the L-sections defining the broad wall of the waveguide and extend into the waveguide squeeze section, i.e., into the waveguide cavity. Each probe was terminated in a dipole radiator (not shown) which acted in conjunction with the other dipole radiators to form the microwave beam. The Eagle scanning technique has been described in the following references, the disclosures of which are incorporated herein by reference: MIT Radiation Laboratory Series, Book 26- "Radar Scanners and Radomes", McGraw-Hill, 1948; L. W. Alverez, "Microwave Linear Radiators", Radiation Laboratory Report No. 366, Jul. 31, 1942; and R. M. Robertson, "Variable Width Waveguide Scanners for Eagle and GCA", Radiation Laboratory Report No. 840, Apr. 30, 1946.
A fundamental problem with the waveguide squeeze technique and, in particular, the Eagle scanner described above is the variation in the amount of coupling to the waveguide probes as the a-dimension is changed. In the Eagle scanner described above, the effective coupling may vary by a factor of 10:1 as the a-dimension is changed for a practical range of operating values. This results in poor scanning performance.
Second, the probe coupling technique used on the Eagle scanner is not practical at millimeter wave frequencies because of the precision required to manufacture, install and adjust the individual probes.